Glow discharge decomposition is employed for the preparation of thin films of a variety of materials such as semiconductor materials, insulating materials, optical coatings, polymers and the like. In a typical glow discharge deposition, a process gas which includes a precursor of the material being deposited, is introduced into a deposition chamber, typically at subatmospheric pressure. Electromagnetic energy, either AC or DC is introduced into the chamber and energizes the process gas so as to create an excited plasma therefrom. The plasma decomposes the precursor material and deposits a coating on a substrate maintained proximate the plasma region. Frequently the substrate is heated to facilitate growth of the deposit thereupon. This technology is well known in the art. Early glow discharge depositions employed either direct current, low frequency alternating current or radio frequency alternating current to energize the plasma; radio frequency current is still very widely employed for this purpose.
One particular drawback to glow discharge deposition processes was their relatively low speed and in an attempt to increase deposition rates those of skill in the art turned to the use of microwave energized plasmas. The earliest microwave energized depositions were carried out in pressure ranges typical of those employed for radio frequency energized deposits, that is to say, pressure ranges of approximately one torr. These depositions were generally not practical for semiconductor materials. While the rate of deposition was quite high, the materials produced thereby were distinctly inferior to those produced with radio frequency energy. The early prior art microwave deposited materials tended to include large numbers of polymeric inclusions and other deviant morphologies; hence, these materials typically were not suitable for use in electrical or optical devices.
In a second generation of microwave energized, glow discharge decomposition processes it was recognized that deposition gas pressures signficantly lower than those employed in radio frequency energized depositions should be employed. These second generation microwave deposition techniques are disclosed in U.S. Pat. Nos. 4,504,518; 4,517,223; 4,701,343 and 4,745,000, the disclosures of which are incorporated herein by reference. In determining the appropriate pressure conditions, the inventors named in the foregoing patents determined the Paschen curve for various glow discharge deposition processes and noted that the curve shifted to low pressures for depositions carried out with microwave energy. As is known to those of skill in the art the Paschen curve is determined by graphing the minimum voltage required to sustain a plasma of a particular process gas in a particular deposition apparatus at various pressures. It is to be noted that in some instances instead of voltage, the graph depicts the minimum power required to sustain a plasma and as such is referred to as a modified Paschen curve. In either instance, the term "Paschen curve" as used herein will refer to both types of graph. In a typical Paschen curve it will be noted that as pressure decreases the power or voltage required to sustain the plasma also decreases to a minimum after which the further decrease in the process gas pressure necessitates an increased power or voltage to sustain the plasma.
As detailed in the above referenced patents, it was the standard prior art practice to carry out depositions at a power-pressure point which approximated the minimum of the Paschen curve. Semiconductor materials prepared by depositions at or near the Paschen curve minimum manifest fairly good electrical and optical properties and can be prepared at relatively high deposition rates. In general though it has been found that materials prepared in a microwave plasma are not always of as high a quality as corresponding materials prepared by a radio frequency plasma, glow discharge deposition. For example, when silicon alloy materials prepared by prior art microwave deposition processes are incorporated into photovoltaic devices, the overall device efficiency decreases as compared to devices in which the alloy is prepared in a radio frequency plasma.
The present invention breaks with the prior art insofar as it recognizes that thin film materials of improved quality will be prepared if deposition conditions are changed to avoid the Paschen curve minimum. As will be described in further detail hereinbelow, it has specifically been found that the quality of semiconductor alloy materials is significantly improved if deposition is carried out at process pressures below the pressure of the minimum point on the Paschen curve and at a power which lies above the Paschen curve at that process pressure.
These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow: